Numerical investigation of the effect of wall-wetting on hydrocarbon emissions in engines

The latest development of direct injection strategy for small gasoline and diesel engines produces significant wall-wetting. It becomes necessary to develop a better understanding of the relationship between wall-wetting and hydrocarbon (HC) emissions. To simulate these emissions from practical fuels, a multi-dimensional code for the spray and combustion was utilized. It included the spray/wall impingement model and the multi-component droplet and film vaporization model for gas and liquid phase transport processes. To extend these models to simulate combustion, a multi-component fuel combustion model was developed, which combined the rate constants for each of the fuel components on the basis of mole fraction to form an effective fuel. The effective fuel was then utilized to compute the reaction rates. The resulting models were then used to simulate the hydrocarbon emissions resulting from wall-wetting in gasoline engines. Qualitative comparisons between the computed and measured results were made. The agreement of both the overall trends and the transient history were reasonable. The computed results also provided significant insight to the causes of HC emissions. Wall-wetting location had a significant effect on HC emissions by allowing some of the unburned HC easy access to the exhaust, while making it nearly impossible for others to escape into the exhaust. The wetting of the cylinder liner underneath the exhaust valves arid the piston top led to large increases in HC emissions. Original is an abstract.